SOx CAPTURE USING CARBONATE ABSORBENT

ABSTRACT

A desulfurization gas process includes water vapor, CO 2  and SO x  (x=2 and/or 3). In a treatment unit, the gas contacts a cooled alkaline aqueous solution having a temperature lower than an initial gas temperature, water and a carbonate of an alkali metal, to cool the gas, condense some water vapor and absorb SO x  in the carbonate-containing solution, produce an SO x -depleted gas and an acidic aqueous solution including sulfate and/or sulfite ions. The SO x -depleted gas and a portion of the acidic aqueous solution can then be withdrawn from the treatment unit. Carbonate of the alkali metal can be added to remaining acidic aqueous solution to obtain a made-up alkaline aqueous solution. This solution can be cooled and reused as the cooled alkaline aqueous solution. An SO x  absorbent solution includes a bleed stream from a CO 2 -capture process, sodium or potassium carbonate, and an acidic aqueous solution obtained from desulfurization.

RELATED APPLICATION

This application claims priority to United States provisional application No. 62/544,382 filed on Aug. 11, 2017, the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The technical field generally relates to processes and systems for the capture of SO₂ and or SO₃ from gases that are produced by various industrial processes, using carbonate absorbent. The technical field also relates to processes and systems for the capture of SO₂ and/or SO₃ from CO₂-containing gases to be further treated for CO₂ removal, such as from flue gases after the combustion of carbon-based fuels.

BACKGROUND

Gases produced by industrial processes, such as post-combustion flue gases, generally comprise water vapour (H₂O), carbon dioxide (CO₂), nitrogen (N₂) and oxygen (O₂), and may also comprise some other compounds such as acid gases SO_(x), NO_(x), H₂S, depending on the process from which the flue gas originates. “SO_(x)” refers to SO₂ and SO₃, and NO_(x) refers to NO and NO₂. These acid gases are chemically absorbed when contacted with alkaline absorption solutions commonly used in CO₂ capture systems.

CO₂ capture can generally refer to processes such as CO₂ absorption and stripping for pure CO₂ production, CO₂ removal from gases, CO₂ pressure swing adsorption, and others. CO₂ utilization may comprise CO₂ transformation into carbonate based compounds, use for algae growth, biofuel applications in fuel cells, such as molten carbonate fuel cells (MCFC).

For numerous post-combustion CO₂ capture processes, when the flue gas to be treated contains SO_(x) in addition to CO₂, the flue gas is first to be treated to reduce its SO_(x) concentration below a threshold value to avoid adverse impact (e.g., reaction, precipitation, inhibition) which may occur in contact with the absorption solution or the adsorbent, as the case may be, that could result in a decrease of the absorption solution or adsorbent capacity. Conventional SO_(x) removal technologies are used for such a purpose.

These conventional SO_(x) removal technologies can be classified as once-through or regenerable, and both comprise wet or dry processes. The once-through wet technologies use limestone, lime, magnesium-enhanced lime or sea water. Technologies consisting of a dry process involve lime spray drying, duct sorbent injection, furnace solvent injection and circulating fluidized bed. Regenerable wet technologies are based on the use of sodium sulfite, magnesium oxide, sodium carbonate and amine; and regenerable dry processes are based on activated carbon. Such technologies are detailed in Srivastava, R. K., Controlling SO ₂ Emission: A review of Technologies, Report EPA/600/R-00/093, November 2000, 113 pages.

These technologies have been implemented as an auxiliary unit to the CO₂ capture or CO₂ utilization process and there has been no integration between both processes (SO_(x) removal and CO₂ capture and/or CO₂ utilization).

In a CO₂ capture process using an alkaline potassium carbonate solution as the absorbing media, SO_(x) and NO_(x) will be absorbed in the alkaline absorption solution and react with the potassium ions resulting in a loss of the absorption solution capacity and the formation of salt precipitates above a specific threshold concentration. To avoid such efficiency losses or precipitation, SO_(x) and NO_(x) levels in the absorption cycle are controlled by bleeding solution from the main absorption loop. In order to reduce the bleed rate required, SO_(x) or NO_(x) contaminants are removed from the flue gas before the main absorption process in a gas conditioning step which also sets the gas inlet temperature for the absorbing process, in a direct or indirect contact unit. In addition, CO₂ capture processes can generate wastes associated with the degradation of the absorption solution. For example, in the case of a CO₂ capture process that uses amines, such as monoethanolamine (MEA), a side-reaction produces a heat-stable salts as undesirable by-products that need to be disposed of.

Even though the SO_(x) levels are considerably reduced in the gas conditioning step, the residual quantity of the contaminants will be absorbed into the alkaline absorption solutions used for CO₂ capture and will accumulate, eventually resulting in a precipitate, which will consume some of the cations away from the carbonate and reduce the solution's absorption capacity. To compensate for these two impacts, part of the absorption solution is bled off to reduce the circulating solution's concentration in sulfate, nitrite and nitrate ions, and the bled off solution is replaced with a fresh carbonate solution. The solution bleed is then disposed of using normal health, safety and environmental practices. This bleeding and replenishment strategy is used in conventional CO₂ capture processes using amines as absorption compounds. A CO₂ capture process using this common strategy is illustrated in FIG. 1.

Referring to FIG. 1, a flue gas (1) is first cooled down in a cooling unit (2) using a water feed (3). Heated water (4) then leaves the cooling unit. The cooled flue gas is then sent to a CO₂ absorption unit (6) where CO₂ and other acid gases, such as SO_(x) and NO_(x) are absorbed in an absorption solution that flows through the absorption unit (6). The treated CO₂-depleted gas is then released to the atmosphere (7). The absorption solution containing the absorbed CO₂, SO_(x) and NO_(x) (8) is heated using a heat exchanger (9) and then sent to a stripping unit (11) where the CO₂ is desorbed from the solution using temperature and/or pressure swing conditions and released as a recovered CO₂ stream (12) for further processing. The absorption solution leaving the stripping unit, which can be referred to as a regenerated absorption solution (10), is lean in CO₂. However, the lean-absorption solution (10) still contains ions coming from the reactions of SO_(x) and NO_(x) in the absorption solution. These ions will accumulate in the absorption solution over time as it circulates through the absorption unit (6) and the stripping unit (11). In order to control their concentration levels in the absorption solution, a volume of the solution is removed as absorption solution bleed stream (13) and replaced with fresh solution (14). In the case of an absorption solution based on the use of carbonates as absorption compounds, such as the UNO MK3 process, SO_(x) and NO_(x) are recovered as KNO₃ and K₂SO₄ solids in the process and removed from the absorption solution using a hydrocyclone.

The impact of SO_(x) and NO_(x) on the performance of CO₂ capture processes is explained in further detail as follows. SO₂ reacts in aqueous media to form a proton and a bisulfite ion (Equation 1). In an aqueous solution, the bisulfite ion is in equilibrium with a sulfite ion and a proton (Equation 2). SO₃ gas reacts with water to form sulphuric acid (Equation 3). In the presence of oxygen, sulfite ions are oxidized to sulfate ions (Equation 4). Sulfate ions are susceptible to precipitation in carbonate absorption solutions, such as a potassium carbonate solution. Precipitation of the K₂SO₄ salt may reduce performance or jeopardize the CO₂ capture process. Additionally, the K⁺ that reacts to form K₂SO₄ can no longer be used in the capture of CO₂, thus reducing the absorption capacity of the solution.

SO_(2(g))+H₂O↔HSO₃ ⁻+H⁺  Equation 1

HSO₃ ⁻↔H⁺+SO₃ ²⁻  Equation 2

SO^(3(g))+H₂O ↔H₂SO₄  Equation 3

HSO₃ ⁻+0.5O₂↔SO₄ ²⁻+H⁺  Equation 4.

In general, there is typically much more SO₂ in the gas compared to SO₃, and therefore Equations 1 and 2 typically dominate.

NO_(x) are also combustion by-products that may be present in flue gases in various ratios. NO_(x) can be absorbed by the absorption solution and transformed into nitrite/nitrate ions in solution as shown in Equation 5, 6 and 7.

NO_((g))+NO_(2(g))+H₂O↔2NO₂ ⁻+2H⁺  Equation 5

2NO_(2(g))+H₂O↔NO₂ ⁻+NO₃ ⁻+2H⁺  Equation 6

2NO₂+SO₃ ²⁻+H₂O→2H⁺+2NO₂ ⁻+SO₄ ²⁻  Equation 7.

A certain percentage of NO_(x) may be irreversibly absorbed by the absorption solution and accumulates as a salt. As for SO_(x), this accumulation may incur an absorption solution loss which results in a decrease CO₂ capture capacity. Each contaminant (SO_(x) and NO_(x)) has a different rate of reaction with the absorption solution. SO₂, the main SO_(x) constituent, is much more soluble in aqueous solution than NO and NO₂. The Henry constant (in water at 25° C.) for SO₂ is 1.2 M/atm while this constant is at 0.0019 and 0.041 for NO and NO₂ respectively (Durham J. L. et al., Influence of gaseous nitric acid on sulfate production and acidity in rain, Atmos. Environ., 15, 1059-1068, 1981). The kinetics of SO₂ reaction with water is very fast and is considered as instantaneous (Ryuichi Kaji et al., Journal of Chemical Engineering of Japan, Vol. 18 (1985) No. 2, p. 169-172). Another reference in this field is Sander, R., Compilation of Henry's Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry, Version 3, 1999, Max-Planck Institute of Chemistry, Germany. The kinetics for the NO_(x) reactions are far slower. The fact that the NO_(x) molecules need to react with themselves before reacting with water (see Equations 5 and 6) combined with the fact that their concentrations in water are very low makes those reactions much slower. This is exemplified in the document “Cost and Performance Baseline for Fossil Energy Plants” Volume 1a: Bituminous Coal (PC) and Natural Gas to Electricity Revision 3, 2015, DOENETL-2015/1723, p. 76 and p. 98, describing the effect of a CO₂ capture unit (based on the CANSOLV technology) on the emission of SO_(x) and NO_(x) from a pulverized coal power plant. In the case B11 described in this document, the unit reduced the total SO_(x) emissions of the power plant from 0.318 kg/MWh to zero. This same unit has no effect on NO_(x) emissions.

As mentioned above, conventional ways to manage impurities in CO₂ capture processes would be to maintain an adequate SO₄ ²⁻ concentration level below the concentration leading sulfate salt precipitation, which may be done by bleeding and disposing of a portion of the absorption solution and replacing it by an equivalent volume of fresh solution. However, as explained above, the absorption and reaction of SO_(x) is faster and more complete than for NO_(x), resulting is an absorption solution having a SO₄ ⁻² ion concentration close to the precipitation threshold concentration, whereas the concentration of NO_(x) species would be much lower than the precipitation threshold concentration. This leads to the absorption of SO_(x) in the absorption solution predominantly controlling the amount of make-up and bleed of the absorption loop.

There is a need for a technology that further enhances SO_(x) removal from gases produced in industrial processes. There is also a need for a technology that further enhances CO₂ capture from gases produced in industrial processes through proper SO_(x) removal from such gases prior to CO₂ capture.

SUMMARY

In some implementations, there is provided a process for desulfurization of a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the gas having an initial gas temperature, the process comprising: contacting, in a treatment unit, the gas with a cooled alkaline aqueous solution comprising water and a carbonate of an alkali metal and having a temperature lower than the initial gas temperature, thereby causing cooling of the gas, condensation of some water vapour and absorption of the SO_(x) in the carbonate-containing solution, and producing a SO_(x)-depleted gas and an acidic aqueous solution comprising sulfate and/or sulfite ions; recovering the SO_(x)-depleted gas from the treatment unit; purging a portion of the acidic aqueous solution exiting the treatment unit (or any other removal step for removing at least a portion of the sulfate and/or sulfite ions); adding carbonate of the alkali metal to a remaining portion of the acidic aqueous solution exiting the treatment unit to obtain an alkaline aqueous solution; and cooling the alkaline aqueous solution to result in the cooled alkaline aqueous solution.

In some implementations, there is provided a system for removing SO_(x) contained in a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the gas having an initial gas temperature, the system comprising: a treatment unit for contacting the gas with a cooled alkaline aqueous solution comprising water and a carbonate of an alkali metal and having a temperature lower than the initial gas temperature, wherein in the treatment unit the gas is cooled, some water vapour is condensed and the SO_(x) are absorbed in the cooled alkaline aqueous solution; a mixing unit for receiving a first portion of an acidic aqueous solution comprising sulfate and/or sulfite ions recovered from the treatment unit and adding thereto carbonate of the alkali metal so as to obtain an alkaline aqueous solution; a purge line for purging a second portion of the acidic aqueous solution recovered from the treatment unit (or any other removal system for removing at least a portion of the sulfate and/or sulfite ions); and a cooling unit for cooling the alkaline aqueous solution to be returned to the treatment unit as the cooled alkaline aqueous solution.

In some implementations, there is provided a use of an alkaline carbonate-containing solution for desulfurization and cooling of a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, and recovering a cooled SO_(x)-depleted gas, wherein the alkaline carbonate-containing solution has a temperature lower than an initial gas temperature and is obtained by mixing an acidic aqueous solution with a carbonate of an alkali metal, and wherein the acidic aqueous solution comprises sulfate and/or sulfite ions resulting from an absorption of the SO_(x) of the gas in the alkaline carbonate-containing solution.

In some implementations, there is provided a process for removing CO₂ from a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the gas having an initial gas temperature, the process comprising a pre-treatment loop for desulfurizing the gas and recovering a SO_(x)-depleted gas, and an absorption loop for removing CO₂ from the SO_(x)-depleted gas, wherein the process comprises: cooling an alkaline aqueous solution comprising water and a carbonate and bicarbonate of an alkali metal to obtain a cooled alkaline aqueous solution having a temperature lower than the initial gas temperature; contacting the gas with the cooled alkaline aqueous solution in a desulfurization unit of the pre-treatment loop, thereby causing cooling of the gas, condensation of some water vapour and absorption of the SO_(x) in the cooled alkaline aqueous solution, and producing the SO_(x)-depleted gas and an acidic aqueous solution comprising sulfate and/or sulfite ions; purging a first portion of the acidic aqueous solution exiting the desulfurization unit; supplying the SO_(x)-depleted gas which contains CO₂ from the desulfurization unit to a CO₂ capture unit of the absorption loop; in the CO₂ capture unit, contacting the SO_(x)-depleted gas with an absorption solution comprising water and carbonate of the alkali metal, thereby causing absorption of the CO₂ in the absorption solution and producing a CO₂-depleted gas and a carbonate and bicarbonate-rich absorption solution; treating the carbonate and bicarbonate-rich absorption solution in a stripping unit to generate a purified CO₂ gas and recover a carbonate and bicarbonate-lean absorption solution; bleeding a fraction of an absorption solution circulating in the absorption loop (e.g., from the carbonate and bicarbonate-lean absorption solution or from the carbonate and bicarbonate-rich absorption solution) as an absorption solution bleed; and mixing the absorption solution bleed with a second portion of the acidic aqueous solution exiting the desulfurization unit of the pre-treatment loop to produce the alkaline aqueous solution.

In some implementations, there is provided a system for removing CO₂ from a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the gas having an initial gas temperature, the system comprising a pre-treatment loop for desulfurizing the gas and recovering a SO_(x)-depleted gas and an absorption loop for removing CO₂ from the SO_(x)-depleted gas, wherein the pre-treatment loop comprises:

a desulfurization unit for contacting the gas with a cooled alkaline aqueous solution comprising water and a carbonate and bicarbonate of an alkali metal and having a temperature lower than the initial gas temperature, wherein in the desulfurization unit the gas is cooled, some water vapour is condensed and the SO_(x) are absorbed in the cooled alkaline aqueous solution;

a mixing unit for mixing a first portion of an acidic aqueous solution comprising sulfate and/or sulfite ions recovered from the desulfurization unit with an absorption solution bleed recovered from the absorption loop to obtain an alkaline aqueous solution comprising water and the carbonate and bicarbonate of the alkali metal;

a purge line for purging a second portion of the acidic aqueous solution recovered from the desulfurization unit; and

a cooling unit for cooling the alkaline aqueous solution to be returned to the desulfurization unit as the cooled alkaline aqueous solution; and

wherein the absorption loop comprises:

a CO₂ capture unit for contacting the SO_(x)-depleted gas with an absorption solution comprising water and carbonate of the alkali metal and producing a CO₂-depleted gas and a carbonate and bicarbonate-rich absorption solution; and

a stripping unit for treating the carbonate and bicarbonate-rich absorption solution to recover a purified CO₂ gas and generate a carbonate and bicarbonate-lean absorption solution at least a part of which is recirculated back into the CO₂ capture unit at the absorption solution.

In some implementations, there is provided a process for desulfurization of a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the process comprising: contacting, in a treatment unit, the gas with an alkaline aqueous solution comprising water and a carbonate of an alkali metal to cause absorption of the SO_(x) in the alkaline aqueous solution, to produce an SO_(x)-depleted gas and an acidic aqueous solution comprising sulfate and/or sulfite ions; recovering the SO_(x)-depleted gas from the treatment unit; withdrawing the acidic aqueous solution from the treatment unit; purging a portion of the acidic aqueous solution as a purged stream and producing a remaining portion of the acidic aqueous solution; adding carbonate of an alkali metal to the remaining portion of the acidic aqueous solution to obtain a made-up alkaline aqueous solution; and reintroducing the made-up alkaline aqueous solution into the treatment unit.

In some implementations, there is provided a system for removing SO_(x) contained in a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the system comprising: a treatment unit for contacting the gas with an alkaline aqueous solution comprising water and a carbonate of an alkali metal, wherein in the treatment unit some water vapour is condensed and the SO_(x) are absorbed in the alkaline aqueous solution to form an acidic aqueous solution comprising sulfate and/or sulfite ions; a mixing unit for receiving a first portion of the acidic aqueous solution recovered from the treatment unit and for adding thereto carbonate of the alkali metal so as to obtain a regenerated alkaline aqueous solution; a purge line for purging a second portion of the acidic aqueous solution recovered from the treatment unit; and a recirculation line for feeding the regenerated alkaline aqueous solution back into the treatment unit.

In some implementations, there is provided a use of an alkaline carbonate-containing solution for desulfurization of a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, and recovering a cooled SO_(x)-depleted gas, wherein the alkaline carbonate-containing solution is obtained by mixing an acidic aqueous solution with a carbonate of an alkali metal, and wherein the acidic aqueous solution comprises sulfate and/or sulfite ions resulting from an absorption of the SO_(x) of the gas in the alkaline carbonate-containing solution.

In some implementations, there is provided a process for removing CO₂ from a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the process comprising a pre-treatment loop for desulfurizing the gas and recovering a SO_(x)-depleted gas, and an absorption loop for removing CO₂ from the SO_(x)-depleted gas, wherein the process comprises: contacting the SO_(x)-depleted gas with an absorption solution comprising water and carbonate of the alkali metal, thereby causing absorption of the CO₂ in the absorption solution and producing a CO₂-depleted gas and a carbonate and bicarbonate-rich absorption solution; treating the carbonate and bicarbonate-rich absorption solution in a stripping unit to generate a purified CO₂ gas and recover a carbonate and bicarbonate-lean absorption solution; bleeding a fraction of the absorption solution circulating in the absorption loop as an absorption solution bleed; supplying at least a portion of the absorption solution bleed to the pre-treatment loop as part of a desulfurization solution; contacting the gas with the desulfurization solution in a desulfurization unit of the pre-treatment loop, thereby causing absorption of the SO_(x) in the alkaline aqueous solution, and producing the SO_(x)-depleted gas and an acidic aqueous solution comprising sulfate and/or sulfite ions; purging a first portion of the acidic aqueous solution exiting the desulfurization unit; supplying the SO_(x)-depleted gas which contains CO₂ from the desulfurization unit to a CO₂ capture unit of the absorption loop; mixing the second portion of the acidic aqueous solution exiting the desulfurization unit of the pre-treatment loop with at least a portion of the absorption solution bleed, to produce the desulfurization solution.

In some implementations, there is provided a system for removing CO₂ from a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the system comprising a pre-treatment loop for desulfurizing the gas and recovering a SO_(x)-depleted gas and an absorption loop for removing CO₂ from the SO_(x)-depleted gas, wherein the pre-treatment loop comprises:

a desulfurization unit for contacting the gas with an alkaline aqueous solution comprising water and a carbonate and bicarbonate of an alkali metal, wherein in the desulfurization unit SO_(x) are absorbed in the alkaline aqueous solution to produce an acidic aqueous solution;

a mixing unit for mixing a first portion of the acidic aqueous solution comprising sulfate and/or sulfite ions recovered from the desulfurization unit with an absorption solution bleed recovered from the absorption loop to obtain an alkaline aqueous solution comprising water and the carbonate and bicarbonate of the alkali metal; and

a purge line for purging a second portion of the acidic aqueous solution recovered from the desulfurization unit; and

wherein the absorption loop comprises:

a CO₂ capture unit for contacting the SO_(x)-depleted gas with an absorption solution comprising water and carbonate of the alkali metal and producing a CO₂-depleted gas and a carbonate and bicarbonate-rich absorption solution; and

a stripping unit for treating the carbonate and bicarbonate-rich absorption solution to recover a purified CO₂ gas and generate a carbonate and bicarbonate-lean absorption solution at least a part of which is recirculated back into the CO₂ capture unit at the absorption solution.

In some implementations, there is provided a use of a bleed stream obtained from a CO₂-capture process and comprising sodium or potassium carbonate for desulfurization of a gas comprising water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3 and production of an SO_(x)-depleted gas.

In some implementations, there is provided an SO_(x) absorbent solution comprising: a bleed stream obtained from a CO₂-capture process and comprising sodium or potassium carbonate; and an acidic aqueous solution obtained from desulfurization of a SO_(x)-containing gas. In some implementations, there is provided a gas pre-treatment solution for absorbing contaminants from a CO₂-containing gas, comprising: a bleed stream obtained from a CO₂-capture process and comprising sodium or potassium carbonate; and an acidic aqueous solution obtained from desulfurization of a SO_(x)-containing gas. The SO_(x) absorbent solution and/or the gas pre-treatment solution can have one or more feature as described herein.

In some implementations, there is provided a process for desulfurization of a gas comprising at least water vapour, CO₂ and SO_(x), where x is equal to 2 and/or 3, the process comprising: contacting, in a treatment unit, the gas with an alkaline aqueous solution comprising water and a carbonate of an alkali metal to cause absorption of the SO_(x) in the alkaline aqueous solution, to produce an SO_(x)-depleted gas and an acidic aqueous solution comprising sulfate and/or sulfite ions; withdrawing the SO_(x)-depleted gas from the treatment unit; withdrawing the acidic aqueous solution from the treatment unit; removing at least a portion of the sulfate and/or sulfite ions from the acidic aqueous solution to produce a remaining acidic aqueous solution; adding carbonate of an alkali metal to the remaining acidic aqueous solution to obtain a made-up alkaline aqueous solution; and reintroducing the made-up alkaline aqueous solution into the treatment unit.

In some implementations, the removing of at least a portion of the sulfate and/or sulfite ions from the acidic aqueous solution can comprise purging a portion thereof to produce a remaining portion thereof as the remaining acidic aqueous solution. In addition, it is noted that where such purging or use of a purge line is mentioned herein it could be feasible alternatively or additionally perform other methods to deplete the acidic aqueous solution of sulfate and/or sulfite ions.

It should be noted that any of the features described above and/or herein can be combined with any other features, processes and/or systems described herein, unless such features would be clearly incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) is a process diagram representing a conventional CO₂ capture process configuration where the flue gas is cooled down using a quench unit and then sent to the CO₂ capture unit. The absorption solution is bled to maintain adequate performance of the unit and fresh absorption solution is added to compensate for the bleed stream.

FIG. 2 is a process diagram representing a process for SO_(x) capture from a flue gas according to an embodiment.

FIG. 3 is a process diagram representing a process for treating a flue gas showing the integration of a pre-treatment loop wherein the gas is cooled and desulfurized, with a CO₂ capture loop for removing CO₂ from the desulfurized gas, according to an embodiment.

FIG. 4 is a process flow diagram of a process for removing CO₂ from a flue gas based on the use of a potassium carbonate solution as the absorption solution.

FIG. 5 is a process flow diagram representing the SO_(x) removal pre-treatment loop used for process simulations, according to an embodiment.

FIG. 6 is a process flow diagram representing a process for treating a flue gas showing the integration of the SO_(x) removal pre-treatment loop with the CO₂ capture loop, according to an embodiment.

FIG. 7 is a graph representing the impact of column height and pre-treatment loop flow rate on the SO_(x) capture. Process conditions are: absorption solution bleed flow rate of 0.092 m³h⁻¹, column flooding maintained at 70%, absorption solution bleed at 17 wt % K₂CO₃, temperature of the pre-treatment loop flow at 30° C.

FIG. 8 is a graph representing the effect of the pre-treatment loop flow temperature on SO_(x) capture and absorption solution bleed flow rate. Process conditions are: column height of 5 m; column flooding maintained at 70%; constant absorption solution bleed flow rate of 0.092 m³h⁻¹; pre-treatment loop flow rate of 200 m³h⁻¹; inlet gas SO₂ concentration of 10 ppmv; absorption solution bleed at 17 wt % K₂CO₃.

DETAILED DESCRIPTION

In a first aspect, the present process and system relates to the treatment of a gas comprising at least water vapour, CO₂ and SO_(x), such as a flue gas, e.g. a post-combustion flue gas, to remove the SO_(x) from the gas while cooling the gas in a single treatment unit. In a second aspect, the process and system relate to a treatment for removing CO₂ from a gas comprising at least water vapour, CO₂ and SO_(x), involving a pre-treatment for removing SO_(x) from the gas, before the CO₂ capture.

In the present description, the treatment unit for SO_(x) removal may interchangeably be referred to as desulfurization unit, quench unit or quench tower. In one embodiment, the quench tower may consist in a contactor such as a packed column with random packing, a packed column with structured packing, a venturi, a barometric leg, an eductor, a spraying device, a demister pad, etc.

According to the present process and system, SO_(x) represents the species SO₂ and SO₃. Water vapour represents water in gaseous form. The gas to be treated may further comprise nitrogen (N₂), oxygen (O₂), NO, NO₂, and/or H₂S, depending on the process from which the gas originates. In one embodiment, the gas may present a concentration in SO_(x) of from about 10 to about 100 ppmv. In some cases, the gas may further comprise NO_(x). (x′=1 and/or 2). In one embodiment, the concentration in NO_(x) in the gas may be of from about 10 to about 150 ppmv. In another embodiment, the gas may also comprise N₂, O₂ and/or other species including for example solid particles.

The term “about”, as used herein before any numerical value, means within an acceptable error range for the particular value as determined by one of ordinary skill in the art. This error range may depend in part on how the value is measured or determined, i.e. the limitations of the measurement system. It is commonly accepted that a 10% precision measure is acceptable and encompasses the term “about”.

A process and system for SO_(x) capture from a combustion or flue gas according to the first aspect will now be described referring to FIG. 2. The combustion or flue gas (1) comprising at least H₂O (in gaseous form), CO₂ and SO_(x) (x=2 and/or 3) is fed to a quench tower (2′) in which it is contacted with an aqueous solution comprising a carbonate of an alkali metal (22).

In some embodiments, the aqueous solution comprising the alkali metal carbonate (22) may comprise sodium carbonate (Na₂CO₃) or potassium carbonate (K₂CO₃). In some embodiments, the alkali metal carbonate comprises K₂CO₃. The aqueous solution of the alkali metal carbonate (22) may further comprises the corresponding bicarbonate of the alkali metal. Hence, if stream (22) comprises K₂CO₃ as the alkali metal carbonate, it may also comprise potassium bicarbonate KHCO₃. The aqueous solution of the alkali metal carbonate (22) optionally comprising the corresponding alkali metal bicarbonate species, may be interchangeably referred to as an alkaline aqueous solution or an alkaline carbonate-containing solution. In one embodiment, the pH of the alkaline aqueous solution may be above 7 and up to about 9.5, or it may be above 7 and up to about 9. In another embodiment, the concentration in alkali metal carbonate in alkaline aqueous solution (22) may be from about 1 mM to about 1 M (the unit “M” corresponding to mol L⁻¹). In some embodiments, the concentration in alkali metal carbonate in alkaline aqueous solution (22) can be from about 1 mM to about 700 mM, 5 mM to about 500 mM, or 50 mM to about 250 mM.

The alkaline aqueous solution (22) sent to the quench tower for being contacted with the gas, generally has a temperature below the temperature of the gas. A cooling unit (18) may thus be provided upstream the quench tower (2′) to cool the alkaline aqueous solution (22). Cooling unit (18) may comprise a heat exchanger in which cooled water is used as cooling fluid (streams 20, 21). In one embodiment, the temperature of the cooled alkaline aqueous solution may be from about 5° C. to about 60° C. In another embodiment, the cooled temperature alkaline aqueous solution can be from about 5° C. to about 50° C. In a further embodiment, the temperature of the cooled alkaline aqueous solution may be from about 10° C. to about 50° C. In another embodiment, the cooled alkaline aqueous solution may have a temperature that is from about 0° C. to about 200° C., from about 20° C. to about 150° C., or from about 50° C. to about 100° C., below the temperature of the gas.

The cooled alkaline aqueous solution (22) exiting the cooling unit is then fed to the quench tower (2′). In the quench tower (2′), the contact between the flue gas and the alkaline aqueous solution (22) results in a lowering of the gas temperature, removal of some water from the gas through water vapour condensation in the solution, as well as removal of the SO_(x) from the gas by absorption in the alkaline aqueous solution. The contact between the flue gas and the solution (22) results in a clean, cooled SO_(x)-depleted gas (5) which may be sent to further treatment as required. For example, the cooled SO_(x)-depleted gas (5) may be sent to a CO₂ capture process to further remove CO₂ therefrom, as will be explained below in connection with the second aspect.

The solution (17) leaving the quench tower (2′) thus contains absorbed SO_(x) in the form of sulfate (SO₄ ²⁻) and/or sulfite (SO₃ ²⁻) ions and to some extent condensed water vapour. The pH of solution (17) is thus acidic, below 7. Solution (17) may thus be referred to as an acidic aqueous solution. Sulfate ions may be susceptible to precipitation in potassium carbonate absorption solution (where the alkali metal is K). However, because the water vapour condenses in the solution, the dilution effect may make precipitation unlikely. Solution (17) may also contain absorbed particles and/or ashes transported by the gas in the quench tower. It may be desirable to remove such particles and/or ashes. This may be done for example at the exit of the quench tower using a separator device (not shown in FIG. 2). In one embodiment, the separation device may comprise a radial vane separator, a Schoepentoeter device, a cyclone, venturi, a settling system, a filtration unit, etc. Additional treatment of the solution (17) can also be performed prior to recycling back into the quench tower (2′) to modify its composition or other properties.

Still referring to FIG. 2, the acidic aqueous solution (17) is then sent to a mixing unit (19) wherein it is mixed with some alkali metal carbonate and optionally the corresponding bicarbonate (15). The alkali metal carbonate may be added to the acidic aqueous solution (17) in a solid form or in solution in water. In one embodiment, the alkali metal carbonate is added to the acidic aqueous solution, in solution in water. In another embodiment, the solution in water of the alkali metal (15) may be derived from a CO₂-capture process, for example it may be an absorption solution bleed derived from a CO₂-capture process, as will be further detailed below in connection with the second aspect. Upon mixing the acidic solution (17) with the alkali metal carbonate, the pH of the solution is increased above 7, resulting in the alkaline aqueous solution (22), which is then returned to the cooler (18) and then the quench tower (2′) for further gas desulfurization. Regarding the mixer (19), it should be noted that it can have various constructions and configurations, such as a Tee pipe joint, a stirred tank mixer, or various other types of mixers, depending on the form of the make-up alkali metal carbonate being added as well as other process factors.

The desulfurization system also includes a purge line (16) that is provided for purging a portion of the acidic aqueous solution (17) exiting the quench tower (2′), so as to maintain a mass balance of the desulfurization process. In one embodiment, purging may be performed at a flowrate determined by a water vapour condensation rate and an alkaline aqueous solution flowrate. A level sensor (not shown) may further be provided upstream of the purge line to detect the level of liquid in the system and allow controlling the purge line flow. The purging of a portion of the acidic aqueous solution may be performed continuously or periodically.

A process and system for CO₂ capture from a combustion or flue gas involving a pre-treatment for removing SO_(x) from the gas, according to the second aspect will now be described referring to FIG. 3.

In FIG. 3, the pre-treatment loop (A) wherein the gas is cooled and desulfurized, substantially corresponds to the desulfurization process/system previously described with respect to the first aspect. Hence, the features of the various streams and units previously described in reference to FIG. 2 may also be considered in relation with pre-treatment loop (A) of FIG. 3. This includes, for instance, the features of the components in the streams, their temperatures, concentrations, pH etc., as well as the features of the quench unit, mixing unit, separation device, cooling unit, level sensor etc.

The pre-treatment loop (A) thus comprises sending gas (1) comprising water vapour, CO₂ and SO_(x), and optionally other species such as NO_(x), to the quench unit (2′). In the quench unit (2′) the gas is cooled to a temperature suitable for the subsequent CO₂ capture unit, some water vapour is condensed as a liquid stream, and a fraction of the SO_(x) present in the gas is removed. Cooling of the gas and SO_(x) removal may be obtained through contacting the gas with alkaline aqueous solution (22), which has been cooled in cooling unit (18) prior to its entrance in the quench unit (2′). As a result of contacting the flue gas, the alkaline aqueous solution is warmed, and its water content is increased due to the water vapour condensation. In addition, its pH is decreased due to the absorption of the SO_(x) in the solution. The acidic aqueous stream (17) leaving the quench unit is sent to mixing unit (19) where it is mixed with absorption solution bleed stream (15) of the absorption loop (B), also referred to as CO₂ capture loop. The absorption solution bleed stream (15) comprises alkali metal carbonate and bicarbonate in solution. Hence, upon mixing the acidic aqueous stream (17) with the bleed stream (15), the pH of the resulting solution increases, resulting in an alkaline aqueous solution, which is returned as stream (22) towards cooler (18). The SO_(x)-depleted gas (5) leaving the quench unit with a lower temperature and a lower SO_(x) concentration is sent to the absorption loop (B). The mass balance of the pre-treatment loop (A) may be obtained using the purge line (16). In one embodiment, purging may be performed at a flowrate determined by a water vapour condensation rate and a flowrate of absorption solution bleed (15). It should be noted that the SO_(x)-depleted gas (5) could also be subjected to additional pre-treatments prior to entering the absorption loop (B), if desired.

The absorption solution bleed stream (15) can be taken from various points in the absorption loop (B). In FIGS. 3 and 6 the absorption solution bleed stream (15) is taken off of the regenerated solution exiting the bottom of the stripping unit. However, the absorption solution bleed stream could be taken from other streams of the absorption loop (B), such as the absorption solution (10) exiting the heat exchanger (9) and which is thus cooler than the regenerated solution exiting the bottom of the stripping unit. The absorption solution bleed stream could be taken from a CO₂-loaded stream, such as the loaded stream (8) exiting the absorption unit or the loaded stream exiting the heat exchanger prior to entering the stripping unit (11). Thus, the absorption solution bleed stream can be taken from the loaded or lean streams in the absorption loop, or could be a combination of such streams.

The absorption loop (B) as shown in FIG. 3, may comprise an absorption unit (6) in which the SO_(x)-depleted gas (5), which contains CO₂, may be treated for removing CO₂ therefrom. Conventional absorbers known in the field may be used as the absorption unit (6). For instance, the CO₂ absorption unit may be a bubble column, a spray scrubber, a packed column with structured packing or a packed column with random packing, a rotating packed bed, or others. In the absorption unit (6), CO₂ is absorbed in an absorption solution comprising water and a carbonate of an alkali metal resulting in the production of a CO₂-depleted gas (7) and a carbonate and bicarbonate-rich absorption solution (8) or simply “rich absorption solution” (8). The flue gas with a lower CO₂ content (7) is discharged at the top of the absorption unit to the atmosphere or it is sent to another downstream process.

In one embodiment, the alkali metal may be sodium or potassium. If the alkali metal is potassium, the absorption solution would thus comprise potassium carbonate and water. In one embodiment, the absorption solution used in the CO₂ capture unit may have a concentration of from about 1 M to about 5 M. It is also noted that the potassium carbonate absorption solution could include additional chemical compounds and/or catalysts. The additional chemical compounds and/or catalysts can include promoters that accelerate CO₂ absorption and/or provide other benefits to absorption or desorption performance. Examples of additional chemical compounds that can be present in the absorption solution are amino acids or salts or derivatives thereof, amines, inorganic additives, and/or other absorbent promoters.

In another embodiment, the CO₂ capture in the absorption unit (6) may be performed in the presence of an enzyme catalyzing the CO₂ hydration such as a carbonic anhydrase (CA) or an analogue thereof. Hence, in the absorption unit (6), CO₂ may dissolve in the absorption solution, which may contain the CA, and then reacts with the hydroxide ions (Equation 8) and water (Equations 9 and 10). The CA-catalyzed CO₂ hydration reaction (Equation 10) is the dominant reaction in the process.

$\begin{matrix} \left. {{CO}_{2} + {OH}^{-}}\rightarrow{HCO}_{3}^{-} \right. & {{Equation}\mspace{14mu} 8} \\ \left. {{CO}_{2} + {H_{2}O}}\rightarrow\left. {H_{2}{CO}_{3}}\rightarrow{{HCO}_{3}^{-} + H^{+}} \right. \right. & {{Equation}\mspace{14mu} 9} \\ {{CO}_{2} + {H_{2}O\mspace{14mu} \mspace{14mu} {HCO}_{3}^{-}} + {H^{+}.}} & {{Equation}\mspace{14mu} 10} \end{matrix}$

The carbonic anhydrase which may be used to enhance CO₂ capture, may be from human, bacterial, fungal or other organism origins, having thermostable or other stability properties, as long as the carbonic anhydrase or analogue thereof can catalyze the hydration of the carbon dioxide to form hydrogen and bicarbonate. It should also be noted that “carbonic anhydrase or an analogue thereof” as used herein includes naturally occurring, modified, recombinant and/or synthetic enzymes including chemically modified enzymes, enzyme aggregates, cross-linked enzymes, enzyme particles, enzyme-polymer complexes, polypeptide fragments, enzyme-like chemicals such as small molecules mimicking the active site of carbonic anhydrase enzymes and any other functional analogue of the enzyme carbonic anhydrase.

The enzyme carbonic anhydrase may have a molecular weight up to about 100,000 daltons. In another embodiment, the carbonic anhydrase can be of relatively low molecular weight (e.g. 30,000 daltons).

The carbonic anhydrase or analogue thereof may be provided in various ways. It may be supported on or in particles that flow with the solution, directly bonded to the surface of particles, entrapped inside or fixed to a porous support material matrix, entrapped inside or fixed to a porous coating material that is provided around a support particle that is itself porous or non-porous, or present as crosslinked enzyme aggregates (CLEA) or crosslinked enzyme crystals (CLEC). The enzymes may be provided immobilized within the reactor itself (e.g., on packing), or may be free in the solution. When the CA is used in association with particles that flow in solution, the enzymatic particles can be prepared by various immobilization techniques and then deployed in the system. When the CA is used in non-immobilized form, it can be added in powder form, enzyme-solution form, or enzyme dispersion form, into the absorption solution where it can become a soluble part of the absorption solution.

In one embodiment, the CA enzyme concentration may be below 0.1% by weight of the absorption solution. In other examples, the CA enzyme concentration can be above this value, depending on various factors such as process design, enzyme activity and enzyme stability.

Still referring to FIG. 3, the carbonate and bicarbonate-rich absorption solution (8) may be fed to a stripping unit (11). Stripping unit (11), which can also be called a desorption unit or a regeneration unit, may serve for the regeneration of the absorption solution and the recovery of the CO₂ as a gas (12). In one embodiment, the carbonate and bicarbonate-rich absorption solution (8) may be heated by passing through a heat exchanger (9) before being sent to the stripper (11). In the stripping unit (11), the rich absorption solution (8) may flow downwards by gravity while contacting a stripping gas which may consist of water vapour at a temperature ranging, for instance, between 60° C. and 85° C. The stripping unit may be operated under a partial vacuum to allow for this low temperature range. A vacuum pump may be used for this purpose. The composition of the stripping gas may be such that the dissolved CO₂ may be released from the liquid phase and consequently bicarbonate ions transformed back into dissolved CO₂ (Equation 11) and then into gaseous CO₂, which may be released as stream (12). The stripping gas is preferably obtained by passing a portion of the solution through a reboiler and generating the stripping gas (see FIG. 6, for example), although other methods of providing a stripping gas can also be used.

CO₂+H₂O→H₂CO₃→HCO₃ ⁻+H⁺  Equation 11.

CA may also be present in the stripping unit and may catalyze the transformation of the bicarbonate ions into dissolved CO₂ (Equation 10). The absorption solution, now made lean in CO₂, leaving the stripping unit and referred to as a carbonate and bicarbonate-lean absorption solution (10) or simply “lean absorption solution” (10) may be pumped and cooled down by passing through the heat exchanger (9) and fed back into the top of the absorption unit (6). A fraction of the carbonate and bicarbonate-lean absorption solution (10) is sent to pre-treatment loop (A) as an absorption bleed stream (15).

Under the complete absorption/stripping cycle, the CA enzyme may be exposed to a pH ranging between about 9 and about 10. The gas (12) leaving the stripping unit, comprising water vapour and gaseous CO₂, may be sent to a condenser. Once condensed, the water may then be sent back to the stripping unit and the CO₂ may be recovered for future use. To maintain the water mass balance, water may be added to the absorption loop (B) through stream (40). When additives (e.g., catalysts such as enzymes, promoters, etc.) are used in the absorption solution, make-up of such additives can be provided via a make-up line at various points in the process.

As previously explained, when the gas to be treated contains NO_(x) species, the NO_(x) can be absorbed by the absorption solution and transformed in nitrite/nitrate ions in solutions as shown in Equations 5, 6 and 7. Over time, the absorption solution in the absorption loop (B) may become richer in nitrite and nitrate ions. The absorption solution bleed (15) flow rate may be adjusted to maintain concentration levels adequate to maintain a continuous optimal CO₂ capture performance.

The present CO₂ capture process and system may present various advantages. First, no separate desulfurization unit may be required for SO_(x) removal prior to CO₂ capture. Indeed, both desulfurization and cooling of the gas can be performed in the same process unit instead of several process units. This allows reducing costs and is economically beneficial. Secondly, the present process/system recycles a waste stream (i.e., the absorption solution bleed stream) to pre-treat the gas and selectively remove contaminants, such as SO_(x). Further, the use of carbonate absorption solution in the main absorption loop can be reduced thanks to the present process/system. These features may reduce the environmental impact of the process and the operating costs associated with the consumption of the absorption solution. Also, removal of SO_(x) from the gas stream in the pre-treatment loop generates sulfates, a desirable by-product which may be used in fertilisers.

The present process and system may also limit the impact of SO_(x) and NO_(x) (if the gas contains such species) on the CO₂ capture performance. Thanks to the pre-treatment step to remove the SO_(x), it may be possible to minimize the sulfate ion concentration in the absorption solution. The nitrate and/or nitrite levels of the absorption loop may further be controlled and reduced. The present process/system also takes advantage of the different absorbing rates of the sulfidation and nitrification reactions in the carbonate absorption solution. More particularly, since the absorption solution bleed at its nitrification threshold point, still has a high absorption capacity, it can be used to selectively absorb sulfates from the gas stream impurities. More specifically, the absorption solution bleed can be mixed or combined with the solution used to cool the flue gas in the quench unit, resulting in a high SO_(x) removal efficiency, which translates into a much lower concentration of sulfate ions in the absorption solution of the CO₂ capture unit, which in turn positively impacts bleed flow rates, enzyme and absorption solution make-up rates by considerably decreasing them. The driving force behind the SO_(x) capture is the pH of the treatment solution in the pre-treatment loop. During standard operation, the aqueous solution is acidic because of the reaction with CO₂, SO_(x) and NO_(x). However, by adding the absorption solution bleed, which has a pH over 9, the pre-treatment loop solution becomes alkaline and may further absorb SO_(x) and NO_(x). This results in a better process performance because the waste disposal and correspondingly the input of fresh solution may be considerably decreased. This reduces the environmental impact of the process and the operation costs associated with the consumption of the absorption solution.

All the documents mentioned in the present description are incorporated herein by reference.

EXAMPLES AND EXPERIMENTATION

The following examples illustrate different aspects of the process and system described herein. For this purpose, Protreat® simulator was used to perform mass and energy balances as well as design of the packed bed columns. Protreat® is a state-of-the-art rate-based simulator for gas treating marketed by Optimized Gas Treating Inc. (OGT) of Houston, Tex. This simulator was implemented with a kinetic module to represent the catalytic effect of carbonic anhydrase's enzyme in a K₂CO₃/KHCO₃ absorption solution on CO₂ capture.

Example 1 (Comparative Example): Description of 100 Tonnes Per Day (Tpd) CO₂ Capture Unit Using an Enzyme Based Solution without SO_(x) Removal

A CO₂ capture process is used to remove 90% of CO₂ present in a flue gas. The flue gas composition is given in Table 1. To take into account the fact that SO_(x) and NO_(x) concentration can vary depending on the flue gas source, their concentrations were changed according to the indicated concentration range in the Table 1. In the present example, SO_(x) consisted of SO₂ only, and SO_(x) removal was considered equivalent to SO₂ removal.

TABLE 1 Inlet Gas Parameters Parameter 100 tpd Case Flow (kg/h) 46-300 Temperature (° C.) 70  Pressure (kPa) 102   H₂O (mol %) 18.0  CO₂ (mol %) 8.2 SO₂ (ppmv*) 10-100 N₂ (mol %) 70.5  O₂ (mol %) 2.5 Ar (mol %) 0.5 NO_(x) (ppmv) 10-150 (*1 ppmv = 1 μL/L) The CO₂ capture process considered for the process simulations is shown in FIG. 4 and is further described below.

The flue gas (1), having a composition shown in Table 1, is directed to a cooling unit (2) having a packed column configuration, using a blower (23). The flue gas is cooled with water at a desirable temperature for the process which is 30° C. for the present example. The water stream leaving the cooling tower is then sent to a cooling system (not shown) and then sent back to the cooling unit (2). The cooled flue gas, at a temperature of 30° C., is then sent to the packed column absorber unit (6). The flue gas enters at the bottom of the packed column and flows upwards and contacts an aqueous absorption solution (25), going downwards by gravity. The absorption solution (25) comprises potassium carbonate, potassium bicarbonate and the enzyme carbonic anhydrase (CA). The potassium concentration in the solution is 2.9 M. The concentrations in carbonate and bicarbonate ions depend on the absorption and stripping process conditions. The CA enzyme concentration is below 0.1% by weight of the absorption solution. CO₂ dissolves in the solution and then reacts with the hydroxide ions (equation 8) and water (equations 9 and 10).

The CA-catalyzed CO₂ hydration reaction (equation 10) is the dominant reaction in the process. The fast enzymatic reaction enables a maximum concentration gradient across the gas/liquid interface and results in a maximum CO₂ transfer rate from the gas phase to the liquid phase, and, consequently in a high CO₂ capture performance. The flue gas with a lower CO₂ content (7) is discharged at the top of the absorber to the atmosphere or it is sent to another downstream process.

Afterwards, the absorption solution containing CO₂ in the form of bicarbonate ions (8), also referred to as the rich absorption solution, is pumped and heated by passing through a heat exchanger (37) and then fed at the top of the stripper (11) as stream (24). The solution flows downwards by gravity while contacting a stripping gas (39) consisting of water vapour at a temperature ranging between 60 and 85° C. The stripper is operated under a partial vacuum to allow for this low temperature range to work, a vacuum pump (35) is used for this purpose. The composition of the stripping gas is such that the dissolved CO₂ is released from the liquid phase and consequently bicarbonate ions are transformed back into dissolved CO₂ (equation 11) and then into gaseous CO₂.

CA is also present in the stripper and catalyzes the transformation of the bicarbonate ions into dissolved CO₂ (equation 10). The absorption solution, now made lean in CO₂, leaves the stripper at its bottom (26). A fraction of the absorption solution is pumped as solution (30) towards the reboiler (27) where water is evaporated and then sent back to the stripper as the stripping gas (39). The energy for water evaporation is provided using waste heat coming from the plant where the capture unit is implemented. Waste heat may for example be supplied using hot water (28) (e.g. at a temperature above 80° C.). The absorption solution (25) is then pumped and cooled down by passing through the heat exchanger (37) and the cooler (38) and is fed back into the top of the absorber unit (6). Under the complete absorption/stripping cycle, the enzyme is exposed to a pH ranging between 9 and 10. The gas leaving the stripper (31), consisting of water vapour and gaseous CO₂, is sent to a condenser (32). Once condensed, the water (33) is then sent back to the stripper and the CO₂ is sent from the vacuum pump (35) to the mechanical compression unit (36) for future use. To maintain the water mass balance, water is added to the process through stream (40).

As the SO₄ ²⁻ concentration level approaches the maximum concentration level, and to avoid any K₂SO₄ precipitation, a fraction of the absorption solution is bled and sent toward the cooling tower (2) as described above. This sulfate concentration level leading to the precipitation is dependent on the composition of the absorption solution and more specifically to the potassium ion concentration in the solution.

Process simulations were conducted to determine the composition of the absorption solution bleed stream required to maintain the SO₄ ⁻² ion concentration level at a maximum concentration of 0.125 M and avoid K₂SO₄ precipitation in a K₂CO₃ absorption solution. The reasons for this were explained above. It was experimentally determined that under the process conditions K₂SO₄ precipitation is present when the sulfate ion concentration is close to 0.15 M.

For a flue gas of Table 1 containing 10 ppmv SO₂ and 10 ppmv NO_(x), the absorption solution bleed composition (stream (13)) were determined for two absorption solutions compositions: 17 and 45 wt % K₂CO₃. Results are provided in Table 2. For both cases, the bleed flow rate is 0.092 m³/h and the bleed flow temperature is 65° C. SO₂ removal in the cooling unit is less than 1% under these conditions.

TABLE 2 Absorption Solution Bleed Composition K₂CO₃ concentration Parameter 17 wt. % 45 wt. % K₂CO₃ (M)  0.87  2.88 KHCO₃ (M)  1.16  3.84 Enzyme (gL⁻¹) 0.5 0.5 SO₄ ²⁻ (M)  0.125  0.125

Example 2: Description of 100 Tonnes Per Day CO₂ Capture Unit Using an Enzyme Based Solution Comprising a SO_(x) Removal in a Pre-Treatment Loop

In the present example, it is considered that a pre-treatment loop is added to the CO₂ capture unit described in Example 1 in the configuration previously shown in FIG. 3. A simple simulation was designed to emulate a pre-treatment loop for absorption solution bleed valuation (see FIG. 5). The complete process is as shown in FIG. 6. For the demonstration purpose, the treatment unit or «quench tower» is a packed column. The quench tower is operated at atmospheric pressure. The liquid is circulated using a centrifugal pump. The liquid flowrate in the purge line is equivalent to the flue gas water vapour condensation rate and the absorption solution bleed flow rate.

As a first simulation, the flue gas of Table 1 was considered, where the NO_(x) and SO₂ concentrations were set at 10 ppmv. This enables a direct evaluation of the impact of the pre-treatment loop on the absorption solution bleed flow rate by comparing with the value obtained in Example 1 which is 0.092 m³/h. As a base case, the simulation was run considering a 5 m quench tower and a pre-treatment loop flow rate of 200 m³/h. The quench tower is operated at 70% flooding and at a temperature of 30° C. so that the flue gas entering the absorber of the absorption loop is at a temperature of 30° C. CO₂ capture process conditions were as described in Example 1 (except for what concerns the use of the bleed stream for SO_(x) removal) where the absorption solution is 17% wt K₂CO₃. The simulations were first started by considering the bleed flow rate determined in Example 1 and were then conducted iteratively until the bleed flow rate converged.

The simulations indicated that by using the absorption solution bleed stream for SO₂ removal, the bleed stream flow rate was decreased from 0.092 down to 0.012 m³/h. This represents an 8-fold decrease of the bleed flow rate relative to the case without SO_(x) treatment when the absorption solution is 17% wt K₂CO₃. As demonstrated in Table 3, the solution entering the cooling unit (2) in Example 1 according to a conventional process, has an acidic pH of 4.42 which does not permit SO₂ capture (less than 1%). However, by adding the absorption solution bled from the absorption loop to the acidic aqueous solution exiting the quench tower (2′), the flow becomes an alkaline solution with pH of 7.78 which promotes SO_(x) capture. In this case, SO₂ removal is 85%.

TABLE 3 Solution inlet to cooling/quench unit when the CO₂ absorption solution is 17 wt % K₂CO₃ and the flue gas contains 10 ppmv NO_(x), and 10 ppm SO₂ Solution composition (mol fraction) Cooling only SO₂ removal Water 1.00E + 00 9.99E − 01 KHCO₃ 1.25E − 04 3.67E − 04 Sulfur Dioxide 1.44E − 05 1.09E − 04 K₂CO₃ 7.39E − 07 5.51E − 04 Enzyme 1.18E − 08 2.37E − 07 Nitrogen 2.92E − 06 6.42E − 06 Nitric Oxide 5.56E − 09 1.96E − 10 Oxygen 1.48E − 06 4.16E − 07 Argon 3.19E − 07 8.97E − 08 Nitrogen Dioxide 9.09E − 10 2.57E − 10 pH 4.42 7.78

In the same system, it was also determined that if instead of having an absorption solution bleed stream at the exit of the stripper (where the solution is CO₂ lean or has a lean loading) one would use the solution at the entrance of the stripper (where the solution is CO₂ rich or has a rich loading), there is no impact on the process water composition and SO₂ removal performance in the quench unit (Table 4). The loading definition is based on the conversion of K₂CO₃ to KHCO₃. A loading of 0.0 will mean that all the carbon is under the form of K₂CO₃. A loading of 0.5, or conversion of 50%, will mean that half the K₂CO₃ has reacted to the KHCO₃ form. A loading of 1.0 will mean that all the carbon is present under the KHCO₃ form. This finding indicates that the bleed stream can be taken from various points in the absorption loop, where the absorption solution may be rich or lean, for desulfurization.

TABLE 4 Impact of CO₂ loading of the absorption solution bleed on the SO_(x) removal performance Lean Rich Loading Loading 0.4 0.7 Quench Tower Diameter (m) 3.1 3.1 CO₂ removal (%)  0.04  0.00 SO₂ removal (%) 86.56 86.56 [K₂CO₃] in solution out quench (mmolL⁻¹)  28.829  28.832 Solution temperature out quench (° C.) 45  45  pH of solution in quench 7.7 7.7

The SO_(x) removal performance of the quench unit was also evaluated for different scenarios by varying:

-   -   Quench tower height;     -   NO_(x) and SO₂ concentration in the flue gas;     -   Temperature of the solution entering the quench unit;     -   Absorption solution bleed flow rate;     -   Potassium carbonate concentration; and     -   Lean or rich absorption solution used for the bleed.

Example 3: Impact of Quench Tower Height and Pre-Treatment Loop Flow Rate (Flow Rate into Quench Unit) on the Performance of the SO₂ Removal Unit Required for a 100 Tpd CO₂ Capture Plant

To determine how the SO_(x) removal unit performance was impacted by quench tower height and pre-treatment loop flow rate, simulations were conducted by varying the column height from 2.5 to 15 m and the pre-treatment loop flow rate from 140 to 500 m³/h. The absorption solution bleed flow rate was set at 0.092 m³/h (value of Example 1) with the bleed composition presented in Table 2 for a 17% wt K₂CO₃ absorption solution at a temperature of 65° C. The pre-treatment loop flow temperature was fixed at 30° C. and quench tower was operated at 70% flooding. The flue gas of Table 1 was considered where the NO_(x) and SO₂ concentrations were set at 10 ppmv. The results are shown in FIG. 7 and Tables 5-8.

TABLE 5 Quench tower of 2.5 m Pre-treatment loop flow rate (m³ · h⁻¹) 140 170 200 230 260 290 350 500 Quench Tower (m) 3.1 3.1 3.2 3.2 3.3 3.3 3.4 3.5 Diameter CO₂ removal (%) 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 SO₂ removal 55.4 58.9 63.6 67.7 71.3 74.4 79.7 88.19 [K₂CO₃] in solution (mmolL⁻¹) 13.3 13.3 13.4 13.5 13.5 13.5 13.4 12.5 out quench Solution temperature (° C.) 69 62 58 54 52 49 46 41 out quench pH solution 7.59 7.53 7.48 7.45 7.42 7.39 7.36 7.28 inlet quench

TABLE 6 Quench tower of 5 m Pre-treatment loop flow rate (m³ · h⁻¹) 140 170 200 230 260 290 350 500 Quench Tower (m) 3.1 3.1 3.2 3.2 3.2 3.3 3.4 3.5 Diameter CO₂ removal (%) 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 SO₂ removal 80.8 84.6 88.2 91.1 93.3 95.0 97.5 99.9 [K₂CO₃] in solution (mmolL⁻¹) 13.3 13.4 13.4 13.5 13.5 13.5 13.4 13.3 out quench Solution temperature (° C.) 69 62 58 54 52 49 46 41 out quench pH solution 7.58 7.50 7.45 7.41 7.38 7.36 7.32 7.27 inlet quench

TABLE 7 Quench tower of 10 m Pre-treatment loop flow rate (m³ · h⁻¹) 140 170 200 230 260 290 350 500 Quench Tower (m) 3.1 3.1 3.2 3.2 3.2 3.3 3.3 3.5 Diameter CO₂ removal (%) 0.03 0.03 0.04 0.04 0.04 0.04 0.04 0.04 SO₂ removal 97.9 99.4 99.9 100 100 100 100 99.95 [K₂CO₃] in solution (mmolL⁻¹) 13.3 13.1 13.4 13.5 13.5 13.4 13.4 13.3 out quench Solution temperature (° C.) 69 62 58 54 52 49 46 41 out quench pH solution 7.57 7.46 7.41 7.37 7.34 7.32 7.29 7.24 inlet quench

TABLE 8 Quench tower of 15 m Pre-treatment loop flow rate (m³ · h⁻¹) 140 170 200 230 260 290 350 500 Quench Tower Diameter (m) 3.1 3.1 3.2 3.2 3.2 3.2 3.3 3.5 CO₂ removal (%) 0.03 0.03 0.04 0.04 0.04 0.04 0.04 0.04 SO₂ removal 99.96 99.95 99.95 99.95 99.95 99.95 99.95 99.95 [K₂CO₃] in solution (mmolL⁻¹) 13.3 13.1 13.4 13.5 13.5 13.5 13.4 13.4 out quench Solution temperature (° C.) 69 62 58 54 51 49 46 41 out quench pH solution inlet quench 7.56 7.44 7.39 7.35 7.32 7.30 7.27 7.23

The simulations demonstrate that it is possible to remove a significant fraction of the SO₂ present in a flue gas, without removing CO₂, using a solution containing a low K₂CO₃ concentration in the range of 13 mmol/L, this is considerably lower than the absorption solution concentration which is 17% wt or 1.45 M K₂CO₃, corresponding to a 110 dilution factor.

The results also show that the SO_(x) removal is influenced by the pre-treatment loop flow rate and by the quench tower height.

Example 4: Impact of the Concentration of NO_(x) And SO_(x) On the Bleed Stream Flow Rate for CO₂ Capture Process Equipped with a SO_(x) Removal Unit. The Absorption Solution in the Absorption Loop is 17% Wt K₂CO₃

The CO₂ capture unit has a 100 tpd capacity and removes 90% of the CO₂ of the flue gas. The flue gas composition is found in Table 1. To determine the impact of NO_(x) and SO₂ concentrations on the bleed stream flow rate, simulations were conducted considering 85% SO₂ removal using a 5 m height quench tower operated at 70% flooding, a pre-treatment loop flow rate of 200 m³/h, a pre-treatment loop flow temperature of 30° C. We assumed no NO_(x) removal in the quench tower under the adopted process conditions. The simulations were run for 2 cases: the first case corresponds to a process without SO_(x) removal and the second case when 85% SO_(x) removal is reached. Results are shown in Table 9.

TABLE 9 Influence of impurities on bleed flow rate [column height of 5 m; column flooding maintained at 70%; pre-treatment loop flow rate of 200 m³h⁻¹; 17 wt % K₂CO₃ absorption solution] Bleed Bleed flow rate flow rate pH K₂CO₃ pre- No pre- SO₂ Pre- Fold solution treatment SO₂ NO_(x) treatment Capture treatment Improvement inlet loop solution (ppmv) (ppmv) (m³h⁻¹) (%) (m³h⁻¹) (—) quench (mmolL⁻¹) 10 10 0.092 86 0.012 8 7.78 30 10 80 0.092 85 0.050 2 7.78 30 10 150 0.092 84 0.092 1 7.78 30 50 10 0.455 86 0.067 7 8.47 138 50 80 0.455 85 0.067 7 8.47 138 50 150 0.455 84 0.092 5 8.47 138 100 10 0.910 86 0.142 6 8.64 250 100 80 0.910 85 0.142 6 8.64 250 100 150 0.910 84 0.142 6 8.64 250

The above results indicate that when conditions are such that the SO₂ concentration and then the SO₄ ²⁻ ion concentration absorbed in solution dictate the bleed flow rates, there is always an important decrease in the flow rate of the bleed stream, namely a decrease of at least of 5-folds However, when the NO_(x) concentration is high, this results in nitrite and nitrate ion concentration levels having a negative impact on the process performance and then determining the bleed flow rate, then the SO_(x) removal unit has no or limited impact. Moreover, it can be observed, for a fixed NO_(x) concentration, that the bleed flow rate is proportional to the SO_(x) concentration in the flue gas. Additionally, the increase in bleed flow rate increases the K₂CO₃ concentration in the process water as well as the pH value. The results on the impact of SO_(x) removal might be different depending on SO_(x) and NO_(x) relative concentrations in the gas to be treated as it can be seen in the above Table.

Example 5: Impact of the Concentration of NO_(x) And SO_(x) On the Bleed Stream Flow Rate for CO₂ Capture Process Equipped with a SO_(x) Removal Unit. The Absorption Solution in the Absorption Loop is 45% Wt K₂CO₃

The CO₂ capture unit has a 100 tpd capacity and removes 90% of the CO₂ of the flue gas. The flue gas composition is found in Table 1. To determine the impact of NO_(x) and SO₂ concentrations on the bleed stream flow rate, simulations were conducted considering using same simulation conditions as Example 4. The conditions are: a 5 m height quench tower operated at 70% flooding, a pre-treatment loop flow rate of 200 m³/h, pre-treatment loop flow temperature of 30° C. We assumed no NO_(x) removal in the quench tower under the adopted process conditions. The simulations were run for 2 cases: the first case corresponds to a process without SO_(x) removal and the second case with SO_(x) removal. The percentage of SO₂ removal was evaluated for each case. Results are shown in Table 10.

TABLE 10 Influence of impurities on bleed rate [column height of 5 m; column flooding maintained at 70%; pre-treatment flow rate of 200 m³h⁻¹; 45 wt % K₂CO₃ absorption solution] Bleed flow Bleed rate flow rate K₂CO₃ No pre- SO_(x) Pre- Fold in process SO₂ NO_(x) Treatment Capture Treatment Improvement water (ppmv) (ppmv) (m³h⁻¹) (%) (m³h⁻¹) (—) pH (mmolL⁻¹) 10 10 0.092 96 0.006 14 8.44 100 10 80 0.092 96 0.050 2 8.44 100 10 150 0.092 96 0.092 1 8.44 100 50 10 0.455 91 0.042 11 8.70 437 50 80 0.455 91 0.050 9 8.70 437 50 150 0.455 91 0.092 5 8.70 437 100 10 0.910 89 0.101 9 8.87 693 100 80 0.910 89 0.101 9 8.87 693 100 150 0.910 89 0.101 9 8.87 693

The above results indicate that when conditions are such that the SO₂ concentration and then the SO₄ ²⁻ ion concentration absorbed in solution dictate the bleed flow rates, there is always an important decrease in the flow rate of the bleed stream, namely a decrease of at least 80%. However, when the NO_(x) concentration is high this results in nitrite and nitrate ion concentrations having a negative impact on the process performance and then having an impact on the bleed flow rate, then the SO_(x) removal unit has no or limited impact. Moreover, it can be observed, for a fixed NO_(x) concentration, that the bleed flow rate is proportional to the SO_(x) concentration in the flue gas. Additionally, the increase in solvent flow rate increases the K₂CO₃ concentration in the process water as well as the pH value. The results on the impact of SO_(x) removal might be different depending on SO_(x) and NO_(x) relative concentrations in the gas to be treated as it can be seen in the above Table.

Example 6: Impact of Pre-Treatment Flow Temperature on SO_(x) Removal Rate and Bleed Flow Rates

Simulations were conducted for a 100 tpd CO₂ capture unit combined with a SO_(x) removal unit in the pre-treatment loop. For the purpose of the simulations the height of the quench tower was 5 m, column flooding was maintained at 70% flooding, pre-treatment flow rate was 200 m³/h, SO₂ concentration was 10 ppmv, the absorption solution for the absorption loop was 1.45 M K₂CO₃ or 17% wt K₂CO₃.

The results indicate that additional benefits can be obtained by operating the pre-treatment loop at higher temperatures (FIG. 8). By increasing the pre-treatment loop flow temperature from 30 to 60° C., the SO₂ removal can be increased from 85 to 100% while the bleed flow rate would be decreased from 0.012 down to 0.006 m³/h. Therefore, the SO_(x) removal pre-treatment may still be integrated in a process where the gas temperature entering the absorber would be higher than 30° C. 

1.-163. (canceled)
 164. A process for removing CO₂ from a gas comprising at least water vapor, CO₂ and SO_(x), where x is equal to 2 and/or 3, the gas having an initial gas temperature, the process comprising a pre-treatment loop for desulfurizing the gas and recovering a SO_(x)-depleted gas, and an absorption loop for removing CO₂ from the SO_(x) depleted gas, wherein the process comprises cooling an alkaline aqueous solution comprising water and a carbonate and bicarbonate of an alkali metal to obtain a cooled alkaline aqueous solution having a temperature lower than the initial gas temperature; contacting the gas with the cooled alkaline aqueous solution in a desulfurization unit of the pre-treatment loop, thereby causing cooling of the gas, condensation of some water vapor and absorption of the Sox in the cooled alkaline aqueous solution, and producing the SO_(x) depleted gas and an acidic aqueous solution comprising sulfate and/or sulfite ions; purging a first portion of the acidic aqueous solution exiting the desulfurization unit; supplying the SO_(x) depleted gas which contains CO₂ from the desulfurization unit to a CO₂ capture unit of the absorption loop; in the CO₂ capture unit, contacting the SO_(x) depleted gas with an absorption solution comprising water and carbonate of the alkali metal, thereby causing absorption of the CO₂ in the absorption solution and producing a CO₂-depleted gas and a carbonate and bicarbonate-rich absorption solution; treating the carbonate and bicarbonate-rich absorption solution in a stripping unit to generate a purified CO₂ gas and recover a carbonate and bicarbonate-lean absorption solution; bleeding a fraction of an absorption solution circulating in the absorption loop (e.g., from the carbonate and bicarbonate-lean absorption solution or from the carbonate and bicarbonate-rich absorption solution) as an absorption solution bleed; and mixing the absorption solution bleed with a second portion of the acidic aqueous solution exiting the desulfurization unit of the pre-treatment loop to produce the alkaline aqueous solution.
 165. The process of claim 164, wherein the gas contacted with the cooled alkaline aqueous solution has a concentration in SO_(x) of from 1 to 00 ppmv.
 166. The process of claim 164, wherein the gas is a flue gas.
 167. The process of claim 164, wherein the gas is a post-combustion flue gas.
 168. The process of claim 164, wherein the temperature of the cooled alkaline aqueous solution is from about 5° C. to about 60° C.
 169. The process of claim 164, wherein the cooled alkaline aqueous solution has a concentration in carbonate of the alkali metal of from about 1 mM to about 1 M.
 170. The process of claim 164, wherein the cooled alkaline aqueous solution has a pH from 7 to 9.5.
 171. The process of claim 164, wherein the desulfurization unit comprises a contacting device selected from a bubble column, a packed column with random packing, a packed column with structured packing, a venturi, a barometric leg, an eductor, a spraying device or a demister pad.
 172. The process of claim 164, wherein the desulfurization unit comprises a packed column with random packing, a packed column with structured packing, or a spraying device.
 173. The process of claim 164, wherein contacting the gas with the cooled alkaline aqueous solution is performed under conditions to produce a SO_(x) depleted gas in which at least 50% and preferably at least 80% of the SO_(x) has been removed.
 174. The process of claim 164, further comprising removing solid particles from the acidic aqueous solution.
 175. The process of claim 164, further comprising removing solid particles from the acidic aqueous solution using a separation system selected from a radial vane separator, a Schoepentoeter device, a cyclone, a settling system or a filtration unit.
 176. The process of claim 164, wherein the purging is performed to maintain a mass balance in the pre-treatment loop.
 177. The process of claim 164, wherein the purging is performed at a flowrate determined by a water vapor condensation rate and an absorption solution bleed flowrate.
 178. The process of claim 164, wherein the gas contacted with the cooled alkaline aqueous solution further comprises NO_(x′) where x′ is equal to 1 and/or
 2. 179. The process of claim 178, wherein the NO_(x′) are present in the gas in a concentration of from 10 to 150 ppmv.
 180. The process of claim 178, further comprising adjusting a flow rate of the absorption solution bleed to maintain a concentration of nitrite and nitrate ions in the absorption loop for optimal CO₂ capture.
 181. The process of claim 164, wherein contacting the SO_(x)-depleted gas with the absorption solution in the CO₂ capture unit is performed in the presence of a carbonic anhydrase or an analogue thereof.
 182. The process of claim 181, wherein the carbonic anhydrase or analogue thereof is present in the absorption solution.
 183. The process of claim 182, wherein the carbonic anhydrase or analogue thereof is in a concentration below 0.1% by weight of the absorption solution.
 184. The process of claim 164, wherein the absorption solution in the CO₂ capture unit has a concentration of from 1 to 5 mol L⁻¹.
 185. The process of claim 164, wherein the alkali metal is potassium or sodium. 